This invention relates to the ion cyclotron range of frequencies (ICRF) heating of plasma with small impurity production. More particularly it relates to the application of an ICRF launcher for providing minimum heating at the edge of the plasma.
Plasma devices, such as those intended for fusion devices, require high temperature plasmas, that is, plasmas including ions moving at relatively high velocity. To achieve such high velocities, energy must be imparted to the ions. One such method of imparting energy is ICRF heating of ions confined in the plasma. In such method, radio frequency (RF) energy is applied at a frequency at which the ions resonate and acquire energy from the applied RF waves.
In toroidal plasma devices ions are constrained by magnetic fields to move in toroidal orbits, hence confining the plasma ions. In such devices a toroidal magnetic field is generated in which the plasma ions follow magnetic lines of flux around the torus. The magnetic lines of flux are twisted to provide equilibrium conditions, but in general the ions spiral around the lines of flux with characteristic periods dependent upon the magnetic field strength and the mass and charge of the ions. It may be assumed that the mass and charge of the species of ions of interest remains constant; however, it is a characteristic of toroidal magnetic fields that magnetic field intensity be stronger toward the major axis of the torus. The consequence of this is that the periods of the ions vary, depending upon their radial displacement from such major axis; hence, the resonant frequency of the ions, the ion cyclotron frequency, varies radially. This results in surfaces substantially circularly cylindrical about tne major axis on which surfaces ions have the same respective ion cyclotron frequency. The surfaces are known as resonance surfaces.
In ICRF heating RF energy is applied at one such radio frequency, usually a frequency for which the resonant surface for the corresponding ion cyclotron frequency intersects the most flux surfaces, a flux surface being a surface of uniform amplitude of magnetic flux. The power of the RF wave is selectively absorbed by the plasma ions at the surface corresponding to such frequency. Here the Doppler shifted phase velocity of the RF wave becomes zero; see, for example, T. H. Stix, The Theory of Plasma Waves, p. 162 (McGraw-Hill, New York, 1962). Typical frequencies for ions of interest are 25 MHz to 60 MHz.
A difficulty with such ICRF heating has been that the level of impurity ions in the plasma has increased dramatically at high ICRF heating power. See J. Hosea et al., PLT Ion Cyclotron Range of Frequencies Heating Program, paper presented at 4th Joint Varenna-Grenoble International Symposium on Heating in Toroidal Plasmas (Rome, March 1984) and K. Odajima, et al., Second Harmonic ICRF Heating Experiment in the JFT--2M Tokamak, ibid.